Pulse generator for use with an electrical musical instrument

ABSTRACT

In an electrical musical instrument of the keyboard type such as an electronic organ having selectively played keys for connecting corresponding tone generator keyers to a source of keying voltage, a voltage differential triggered monostable multivibrator pulse generator operable to produce an electrical pulse upon playing any one of the keys, the pulse generator being especially suitable for operating, simultaneously with key actuations, a unitary instrumental or percussive rhythm voice generator common to all of the keys in order to provide a pleasing musical effect. The pulse generator includes a sensing means in integral circuit arrangement with the keys and the voltage source. Playing any one of the keys serves to produce a voltage differential across the sensing means to trigger the monostable multivibrator pulse generator to produce a pulse output of predetermined duration regardless of the number of keys already being played.

United States Patent Petrie et a1.

[ PULSE GENERATOR FOR USE WI AN ELECTRICAL MUSICAL INSTRUMENT [75] Inventors: Adelore I Petrie, Arlington Heights; Wilford R. Schreier, Bensenville, both of I11.

[73] Assignee: Hammond Corporation, Deerfield,

[58] Field of Search 84/10], 1.03, 1.17, 1.24, 84/DIG. 8, DIG. 12, DIG. 22, DIG. 25, 1.13,

1.26, DIG. 23

[56] References Cited I UNITED STATES PATENTS 3,580,980 5/1971 Uetrecht 34/124 X 3,715,445 2/1973 3,719,767 3/1973 Matumoto et a1. 84/101 Kev/M7505 2 l j/ l [11] 3,21,457 1 June 28, 1974.

Primary Examiner-Richard B. Wilkinson Assistant Examiner-Stanley J. Witkowski Attorney, Agent, or Firm-Lowell C. Bergstedt 5 7] ABSTRACT In an electrical musical instrument of the keyboard type such as an electronic organ having selectively played keys for connecting corresponding tone generator keyers to a source of keying voltage, a voltage differential triggered monostable multivibrator pulse generator operable to produce an electrical pulse upon playing any one of the keys, the pulse generator being especially suitable for operating, simultaneously with key actuations, a unitary instrumental or percussive rhythm voice generator common to all of the keys in order to provide a pleasing musical effect. The pulse generator includes a sensing means in integral circuit arrangement with the keys and the voltage source. Playing any one of the keys serves to produce a voltage differential across the sensing means to trigger the monostable multivibrator pulse generator to produce a pulse output of predetermined duration regardless of the number of keys already being played.

5 Claims, 2 Drawing Figures BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to pulse generators for use with electrical musical instruments of the keyboard type operable to produce electrical pulse outputs when any one of the musical instrument keys is actuated.

2. Description of the Prior Art In an electrical musical instrument of the keyboard type such as electronic organ having selectively played keys for connecting corresponding well-known tone generator keyers to a source of keying voltage, it is frequently desirable to obtain an electrical pulse upon playing any one of the keys in order to simultaneously actuate a unitary instrumental or percussive rhythm voice generator common to all of the keys, such as the brush or cymbal, for example, for providing a pleasing musical effect. Generators for such pulses are also used in other electronic organ applications requiring the occurrence of an electrical pulse of predetermined duration upon playing any one of the keys, regardless of how many keys are already being played.

Prior systems known to applicant for producing such pulses generally comprise the combination of a plurality of relatively high resistances connected in series with one relatively low resistance which is in turn connected to ground, each high resistance being switchable by a corresponding key to a bus in circuit with a voltage source. When the value of each such high resistance is much greater than the low resistance (by a ratio on the order of one thousand to one), Playing one or more keys will produce a voltage drop across the single low resistance approximately proportional to the number of keys played, provided that only a few of the keys are simultaneously played; this voltage drop developed across the low resistance is then differentiated bya capacitor to produce a pulse in response to each key actuation.

Such systems have proven inadequate, as those familiar with electrical musical instruments will appreciate, insofar as the incremental voltage drop developed across the low resistance decreases as the number of simultaneously played keys increases and more of the high resistances are thereby connected in parallel, with the result that pulses of smaller amplitude are produced with additional key actuations. Furthermore, a separate set of switch contacts for each key is required for operating such a pulse generator. In addition, key bounce frequently results in the undesirable multiple actuation of a voice generator when only one key is played.-

SUMMARY OF THE INVENTION The present invention overcomes the shortcomings of prior known systems by utilizing a small number of inexpensive electronic components in a circuit operable to produce electrical pulses of uniform amplitude upon playing any of the keys regardless of the number of keys already being played. This circuit is acutated by the same set of key contacts that energize corresponding tone generators and does not employ a large configuration of resistors, with appreciable resulting savings in manufacturing and assembly costs. The pulse output of this circuit does not reflect the undesirable effects of key bounce. Furthermore, this circuit is not actuated by transient voltage variations at its input voltage source.

The present invention comprises a voltage differential triggered monostable multivibrator pulse generator adapted for use with an electrical musical instrument of the keyboard type such as an electronic organ having selectively played keys for connecting corresponding tone generator keyers to a source of keying voltage. The pulse generator-includes a sensing means in integral circuit arrangement with the keys and the voltage source. Playing any one of the keys serves to produce a voltage differential across the sensing means to trigger the monostable multivibrator pulse generator to produce a pulse output of predetermined duration regardless of the number of keys already being played.

The preferred embodiment of the present invention comprises first and second transistors in circuit with each other and having interconnected emitters, a sensing means such as a resistor connecting the voltage source to the keys and the emitters, a capacitor having a first terminal connected to the voltage source, a second resistor for operatively connecting the second terminal of the capacitor to ground potential, the second terminal being connected to the base of the first transistor; and means in circuit with the transistors for biasing them to normally maintain the first transistor'in a conductive state and the second transistor in a corresponding non-conductive state in response to the conduction of the first transistor. Conduction of the first transistor causes voltage at the second terminal to reach a first predetermined level, and playing any one of the keys serves to operably reverse the respective states of the transistors in response to a voltage differential across the sensing means only until voltage on the second terminal changes after a predetermined time interval from the first level to a second predetermined level sufficient to re-initiate conduction of the first transistor, the pulse duration or width being determined by the conduction time of the second transistor. Playing any one of the keys actuates the pulse generator to produce a pulse of duration determined by the time required for voltage on the second terminal to change from the first to the second level. The pulse duration is preferably much less than the time interval between successive key actuations in normal operation of the musical instrument, so that a pulse is produced in response to each key actuation. Transient voltage variations of the voltage source are ineffective to trigger the pulse generator circuit, since they equally affect the base and emitter voltages of the first transistor with no resulting bias change to reverse its state of operation. Other suitable electronic switching means may be employed in place of transistors in the pulse generator circuit.

BRIEF DESCRIPTION OF THE DRAWINGS- FIG. I is a schematic illustration of the preferred embodiment of the pulse generator according to the present invention; and

FIG. 2 is a pictorial representation of the approximate operating voltages appearing at three selected 10- cations in the circuit of FIG. ll for a typical key actuation.

DESCRIPTION OF THE PREFERRED EMBODIMENT With reference to the schematic illustration of FIG. I, the preferred embodiment of the'voltage differential triggered monostable multivibrator pulse generator of the present invention, generally designated at 10, is utilized in an electrical musical instrument of the keyboard type such as an electronic organ having a plurality of selectively played or actuated keys 11 for connecting corresponding well-known tone generator keyers 12 to a keying bus 13 in circuit with a source of keying voltage. The preferred type of keyer is shown in Schrecongost US. Pat. No. 3,748,944. For the purpose of simplification, the tone generators themselves are not shown in FIG. 1, and only a portion of the musical instrument keyboard is illustrated. It should be noted that the pulse generator is actuated by the same set of key contacts that are used to energize corresponding tone generators.

The pulse generator 10 comprises first and second switching means or transistors 14 and 16 in circuit with each other and having respective terminals or emitters l7 and 18 interconnected at a junction 19. A resistor or other suitable sensing means 21 (such as a diode or an inductor) is provided in integral or series circuit arrangement with a voltage source at terminal 22 and the keying bus 13 and the keys 11 by means of leads 23 and 24. The resistor or sensing means 21 is connected to the emitters 17 and 18 through leads 26 and 27 respectively. A capacitor 28 has a first terminal or negative side 29 connected to the voltage source at terminal 22 by a lead 23, and a second terminal or positive side 31 connected to a ground lead 32 by a resistor 33, the second terminal 31 of capacitor 28 being connected to the control element or base 34 of transistor 14 through a lead 36. Operation of switching means or transistor 14 is determined by the voltage level on control element or base 34. The collector 37 of transistor 14 is connected to the base 38 of transistor 16 by a lead 39.

Circuit means are provided for operating or biasing transistors 14 and 16 to normally maintain transistor 14 in a saturated state and transistor 16 in a corresponding non-saturated or cutoff state in response to the saturated state of transistor 14; resistors 41 and 42 are provided for respectively connecting collector 37 of transistor 14 and collector 43 of transistor 16 to ground lead 32. Transistor 14 is normallysaturated because its base 34 is connected through resistor 33 to ground lead 32, and its emitter 17 is connected through resistor 21 to 28 volts, thus producing strong forward bias on its base-emitter junction. When transistor 14 is conducting, the base 38 of transistor 16 is effectively connected or shorted to its emitter 18 through conducting transistor 14 and leads 39, 26 and the result is that transistor 16 is normally non-conducting due to the absence of forward bias on its base 38, i.e. since emitter 18 and base 38 of transistor 16 are essentially at the same potential, the base-emitter junction of transistor 16 is not sufficiently forward biased to turn it on.

The pulse output is determined by the conduction time of transistor'l6, the pulse being obtained at terminals 44 and 46 connected across resistor 42 in series with collector 43.

It should be noted that the voltages and component values illustrated in FIG. 1 correspond to voltages and component values conveniently available and utilized in a typical electronic organ, but operation of the pulse generator 10 of the present invention is not restricted to the values shown, their selection being merely a matter of design choice. The circuit operates equally well with PNP as with the illustrated NPN type transistors,

with appropriate polarity reversals on voltage supplies and components being necessary for PNP transistors.

The operation of the pulse generator 10 according to the present invention will be easily understood with additional reference to FIG. 2, which is a pictorial representation not-to-scale of the approximate operating voltages for a typical key actuation appearing at terminal 19 (FIG. 2a), terminal 31 (FIG. 2b) and at terminals 44 and 46 (FIG. 2c). For the voltages and component values'shown in FIG. 1 with terminal 22 being connected to a source of 28 volts, when transistor 14 is in saturation, the voltage at terminal 19 will be approximately 27.9 volts due to operation of the voltage divider comprising resistors 21 and 41, neglecting the very small voltage drop between emitter l7 and collector 37 of transistor 14 (FIG. 2a). Assuming that the forward drop of the base-emitter junction of transistor 14 is approximately 0.7 volts which is typical of commercially available transistors, the voltage at the second terminal 31 of capacitor 28 will reach a first predetermined level of approximately 27.2 volts in response to conduction of transistor 14 (FIG. 2b).

Playing or actuating any one of the keys 11 will produce a differential voltage drop across resistor 21 caused by additional current drawn through resistor 21 by atone generator keyer 12 through the actuated key. This differential voltage drop triggers monostable multivibrator pulse generator 10 to produce a pulse output of predetermined duration in the following manner. The additional current drawn through resistor 21 when an additional key is actuated causes the voltage on emitter 17 of transistor 14 (and terminal 19) to become less negative. Capacitor 28 keeps the voltage on base 34 of transistor 14 from changing rapidly, and thus transistor 14 begins to come out of saturation. Accordingly the current through resistor 41 decreases, which causes the voltage on base 38 of transistor 16 to be less negative than its emitter 18, and transistor 16 starts to turn on. As transistor 16 turns on, additional current drawn through resistor 42 in its collector circuit and resistor 21 in its emitter circuit cause an additional change in the voltage at terminal 19. The process is regenerative and transistor 14 rapidly goes to cut-off and transistor 16 to saturation. When this occurs, the voltage on terminal 19 is about 25.5 volts due to the voltage divider effect of resistors 21 and 42.

With transistor 14 off the voltage on terminal 31 of capacitor 28 is no longer set by the drop across the base-emitter junctionof transistor 14 and capacitor 28 begins to charge exponentially toward ground potential through resistor 33. When the voltage on terminal 31 (and base 34 of transistor 14) passes the 25.5 volts on terminal 19 and reaches 24.8 volts, transistor 14 begins to turn on, transistor 16 begins to turn off, and through the regenerative process, transistor 14 goes rapidly into saturation and transistor 16 into cut-off. The voltage on terminal 19 rapidly changes back to nearly the original 27.9 volts and the voltage on terminal 31 also changes back to nearly the original 27.2 volts. The steady state current drawn by the keyer, if the key remains actuated, is typically quite small so that the voltages on terminals 19 and 31 will be slightly different at the end of the operating cycle of monostable multivibrator 10. However, if a second key is actuated while the first remains actuated, additional differential current will be drawn through resistor 21 to retrigger monostable multivibrator 10 producing a second pulse.

Each time an additional key is actuated, additional differential current is drawn, and the monostable retriggers.

As shown in FIG. 2C, when transistor 16 turns on and goes into saturation, the voltage at terminal 44 drops from ground volts) to about 25.5 volts and remains there until transistor 14 turns back on to turn off transistor 16. For the circuit parameters given in FIG. 1, the duration of the output negative pulse is about milliseconds.

Where, by proper selection of component values, the pulse duration (10 milliseconds, for example) is much less than the time interval between successive key actuations during normal non-chord operation of the musical instrument, a pulse is produced in response to each key actuation. If two of the keys ll are actuated simultaneously, only one pulse will be produced. Should a key 11 be played while the pulse generator 10 is in the process of producing a pulse in response to a previous key actuation, the first key actuation will predominate with the second key actuation having no effect. Such operation, however, avoids undesirable multiple actuation of a voice generator in response to key contact bounce. By varying the time interval between successive key actuations, the musical instrument operator can selectively operate the pulse generator 10 to produce a separate pulse for each key actuation or only one pulse for each series of multiple key actuations, depending upon the desired musical effect.

It should be noted that transient voltage variations at the voltage source at terminal 22 will not trigger the pulse generator W, as such variations will equally affeet the voltages at the base 34 and emitter 17 of transistor 14 through relatively low impedances (capacitor 28 and resistor 21 respectively) with no resultant bias change to reverse its state of operation.

We claim:

1. In an electrical musical instrument of the keyboard type having a plurality of tone generator keyers, a source of keying voltage, a plurality of selectively actuated keyswitches for connecting associated ones of said tone generators to said source, and a voltage differential triggered monostable multivibrator circuit coupled between said keyswitches and said source to produce an electrical pulse output of predetermined duration each time one of said keyswitches is actuated, said multivibrator circuit including a sensing circuit means connected between said keys and said source to sense differential current drawn by at least one of said tone generator keyers each time at least one of said keyswitches is actuated to trigger said monostable multivibrator circuit regardless of the number of keyswitches already being actuated.

2. Apparatus as claimed in claim ll, wherein said multivibrator circuit comprises a common-emitter type monostable multivibrator and said sensing circuit means comprises a resistor connecting said common emitters and said keyswitches in parallel to said source.

3. In an electrical musical instrument of the keyboard type:

a source of keying voltage;

a keying bus;

a plurality of tone generator keyers;

a plurality of keyswitches connected between said keying bus and said keyers;

a resistor connecting said source to said bus for sensing differential current drawn by one of said keyers each time one of said keyswitches is actuated; and

a monostable multivibrator circuit coupled to said resistor and said source of keying potential for producing an output pulse in response to said sensed differential current in said resistor.

4. In an electrical musical instrument of the keyboard type:

a source of keying voltage;

a keying bus;

a plurality of tone generator keyers;

a plurality of keyswitches connected between said keying bus and said keyers; and

a monostable multivibrator circuit coupled between v said keying bus and said source comprising:

a resistor connected between said source and said keying bus;

a pair of transistors each having emitters connected in common to said keying bus and collectors connected via scaled resistors to a source of ground reference potential and having the base of a second of said transistors coupled to the collector of a first of said transistors;

voltage divider comprising a capacitor and a second resistor with a junction therebetween connected to the base of said first transistor;

said first transistor being in saturation and said second transistor in cut-off during the quiescent state of said multivibrator with said operating states of said transistors being regeneratively reversed each time one of said keyswitches is actuated to produce differential current through said resistor drawn by one of said keyers and thereby to alter the operating voltage on the emitter of said first transistor.

5. Apparatus as claimed in claim 41, wherein said scaled resistors and said second resistor and capacitor have values preselected to produce an output pulse from said multivibrator of a duration less than the normal interval between successive keyswitch actuations. l =l 

1. In an electrical musical instrument of the keyboard type having a plurality of tone generator keyers, a source of keying voltage, a plurality of selectively actuated keyswitches for connecting associated ones of said tone generators to said source, and a voltage differential triggered monostable multivibrator circuit coupled between said keyswitches and said source to produce an electrical pulse output of predetermined duration each time one of said keyswitches is actuated, said multivibrator circuit including a sensing circuit means connected between said keys and said source to sense differential current drawn by at least one of said tone generator keyers each time at least one of said keyswitches is actuated to trigger said monostable multivibrator circuit regardless of the number of keyswitches already being actuated.
 2. Apparatus as claimed in claim 1, wherein said multivibrator circuit comprises a common-emitter type monostable multivibrator and said sensing circuit means comprises a resistor connecting said common emitters and said keyswitches in parallel to said source.
 3. In an electrical musical instrument of the keyboard type: a source of keying voltage; a keying bus; a plurality of tone generator keyers; a plurality of keyswitches connected between said keying bus and said keyers; a resistor connecting said source to said bus for sensing differential current drawn by one of said keyers each time one of said keyswitches is actuated; and a monostable multivibrator circuit coupled to said resistor and said source of keying potential for producing an output pulse in response to said sensed differential current in said resistor.
 4. In an electrical musical instrument of the keyboard type: a source of keying voltage; a keying bus; a plurality of tone generator keyers; a plurality of keyswitches connected between said keying bus and said keyers; and a monostable multivibrator circuit coupled between said keying bus and said source comprising: a resistor connected between said source and said keying bus; a pair of transistors each having emitters connected in common to said keying bus and collectors connected via scaled resistors to a source of ground reference potential and having the base of a second of said transistors coupled to the collector of a first of said transistors; voltage divider comprising a capacitor and a second resistor with a junction therebetween connected to the base of said first transistor; Said first transistor being in saturation and said second transistor in cut-off during the quiescent state of said multivibrator with said operating states of said transistors being regeneratively reversed each time one of said keyswitches is actuated to produce differential current through said resistor drawn by one of said keyers and thereby to alter the operating voltage on the emitter of said first transistor.
 5. Apparatus as claimed in claim 4, wherein said scaled resistors and said second resistor and capacitor have values preselected to produce an output pulse from said multivibrator of a duration less than the normal interval between successive keyswitch actuations. 